1. Field of the Invention
The present invention relates to drill pipe strings and the method for forming the same, and more particularly to providing a lightweight string for extended reach and short radius drilling and to protecting connections for transmitting signal and/or power across the joints forming the pipe string.
2. The Prior Art
Oil well drilling is often performed from drilling platforms, which, for deep water applications may involve costs of several hundred thousand dollars per day. Thus, there is a great demand for drill pipe strings which are lightweight yet durable enough to reach greater distances by directional drilling from the platforms to thereby add to the return on investment. The design discipline for short radius and extended reach are often different but both benefit from favorable strength to weight ratios.
The ability to adjust drilling rate, reach and direction in the pursuit of oil traps has long been rendered inefficient by the lack of a reliable and durable pipe string and method for obtaining feedback during drilling. The depth and reach of exploration are restricted primarily by the cumulative weight and unit strength of a drill pipe string. Steel pipe has been effective for its durability but its weight-to-strength ratios has reached its boundaries in ultra-deep, deep directional and/or extended reach drilling because of its weight or by the weight induced friction of the rotating pipe string as it rests on the walls of the well bore or rubs against the casing wall. The stiffness associated with steel has also made adjusting to or performing short radius turns difficult and dangerous as drill operators are essentially guessing at the changes in subterraneous conditions as the pipe progresses without consistent signal feedback. The drilling entrepreneur, therefore, has been limited for some time by the stiffness, the unit weight, and the fatigue, shear and tensile strength limits of metal pipe.
Furthermore, deep drilling and directional drilling require monitor and feedback of the drilling environment. As a drill string progresses, the rotational velocity, torque, and stress variations require measurement to guide the drill workman in adjusting the drill operation. Measuring while drilling (MWD) also allows the drill operator to periodically check the likelihood of successfully locating an oil trap. The dilemma thus far has been in successfully leading a signal down the pipe. Prior efforts have focused on leading instrumentation or signal lines down through hollow pipe interiors. Signals that are traditionally carried down through the pipe tend to suffer adverse consequences of the rigorous and treacherous environment associated with drilling. As the pipe progresses, chunks of sharp, hard earth and particulates regurgitate back toward the surface along the pipe circumference or through its interior. As a result, various material alternatives have been proposed both to reduce the linear weight and flexure of the string as well as to improve its fatigue, shear and tensile limits and methods proposed to carry a signal between pipe string segments. The search for light weight and high strength material substitutes has led to composite pipe structures since composites also offer the added benefits of being more resistive to corrosion.
Composites, while sufficiently lightweight and durable for deep and directional drilling, are less effective at forming the mechanical joints required in drill pipes. While virtually all drilling operations require limited length pipe segments determined by the size of the drilling rig and/or the handling power of any lifting equipment, the step of joining such pipe segments into a long string is a fundamental aspect of all drilling. For this reason, the more recent development focus has been directed to the interface between the composite wall of the pipe and metal end fittings on a segment end. The interface of these two structures provides an economically and mechanically viable structure for extended reach and short turn radius drilling.
Composite materials have a further advantage that heretofore has not been extensively recognized, namely the convenient imbedding of signal and/or power conductors into the laminates forming the pipe wall. Imbedding the signal within the pipe walls is particularly useful with short radius directional drilling since it allows for an uninterrupted, continuous down hole signal feedback and control augmentation while drilling, thus maximizing the effectiveness of the invariably very high drilling costs at the remaining remote or deeply submerged formations. Additionally, the non-conductive nature of composites insulates electrical signals when used, preventing dissipation of power into the pipe walls. This synergistic aspect of composite pipe has not been fully recognized nor exploited in the art simply because the technical challenge of forming an effective connection between the composite tube wall and the end fitting has overwhelmed all other considerations. The process of imbedding conductors or connecting them across a joint from a metal/composite interface coupled with protecting the signal line down the pipe string length has therefore been relegated to inattention.
At the core is the inherent difficulty in forming a high integrity interface between the composite pipe wall and the adjoining surfaces of the end fitting. In the past, fitting assemblies with variously opposing surface geometries have been proposed to effect a secure capture of the composite end within the fitting. Some examples of such end fittings include those taught in U.S. Pat. No. 5,233,737 to Policelli; U.S. Pat. No. 4,810,010 to Jones; U.S. Pat. No. 6,315,002 to Antal et al.; and others. While suitable for the purposes intended each of the foregoing assemblies include threaded or otherwise releasably engaged parts clamping the composite between each other with inherently uneven load concentrations resulting in highly uneven shear stresses. This uneven load distribution between adjacent parts, of course, results in correspondingly uneven local strain deformations when exposed to the various high loadings in the course of use. There is therefore an inherent incidence of local bond separation between the composite itself and the adjoining fitting surface, with some consequence for failure.
Alternatively, end fitting assemblies have been proposed in which radial pins or other radial fasteners are added to the assembly, as exemplified by the teachings of U.S. Pat. No. 5,332,049 to Tew; U.S. Pat. No. 5,288,109 to Auberon et al.; U.S. Pat. No. 5,443,099 to Chaussepied et al.; and others. Once again, while a change is realized from these radial interconnections the essentially separated nature of a single metal to composite surface interface is also susceptible to uneven load transfer with the consequent local separations an inherent possibility. For example, the '109 patent to Tew appears to disclose a single metal-composite interface held together by radial pins and an adhesive bond which may suffer from disparate torsional forces. Tew appears to propose an outer protective sheath lacking a tapered surface interface and suffers the shortcoming that, particular to long reach applications, the coupling itself fails to provide a high strength joint capable of carrying the high torsional force necessary to withstand the loads of both extended reach applications and short radius. Moreover, such assemblies suffer from a lack of another significant attribute, namely the bridging of a protected electrical signal between pipe connections.
In the past various conductor connection arrangements bridging a pipe joint have been proposed for transmitting power and signals down pipe strings. Examples of such arrangements can be found in the teachings of U.S. Pat. No. 6,367,564 to Mills et al.; U.S. Pat. No. 4,220,381 to van der Graaf, U.S. Pat. No. 2,748,358 to Johnston; and U.S. Pat. No. 5,334,801 to Mohn. Each of these, and others similarly implemented, either refer to indirect coupling like that obtained by capacitive coupling or by Hall effect, or speak of full insulation of paired leads in light of the conductive nature of the pipe string, or incorporate a conductive tube surrounded between two insulative regions as in the '564 patent to Mills. Additionally, where electrical leads are incorporated throughout the pipe string, the signal and power carrying arrangements suffer from being lead down the string either within the pipe hollow passage or external the pipe exposed to damaging objects. Thus while suitable for the purposes intended these prior art teachings do not avail themselves to all the advantages of a composite, non-conducting pipe string and the protective capabilities of embedding a conductor in the composite walls and it is these advantages that are realized herein. Such arrangements are either prone to damage or add significant weight and contribute to inflexibility in the pipe string by incorporating full length metal tubes within the structure.
Pipes has been proposed which include current loop inductive couplers with the electrical cables running along the interior of the pipe sections. A device of this type, as shown in U.S. Pat. No. 6,866,306 to Boyle. The adductive loop coupling has not proven particularly acceptable and the electrical cable exposed on the interior of a pipe subjects the cable to the elements within the pipe and to damage from components extended through the pipe. Furthermore, such pipe devices do not lend themselves to short turn pipe constructions.
It can be seen then that a need exists for a lightweight and durable structure capable of withstanding the rigors of deep and directional drilling that is also capable of carrying a protected signal down a pipe string length.